1. Field of Invention
The present invention relates to an additive for improving early resistance to driving rain of physically curing coating compositions based on aqueous acrylate dispersions and a method of improving early resistance to driving rain of such compositions.
2. Description of the Prior Art
Physically curing coating compositions based on aqueous acrylate dispersions, which contain, as essential components, an aqueous acrylate latex, inorganic fillers and conventional ancillary materials such as defoamers, plasticizers, preservatives, extenders, wetting agents, dispersants and thickeners, are used on a large scale for coating surfaces exposed temporarily to the effects of weather. The strictly physical drying of the coating composition is a disadvantage of such systems, especially when they are used as fire protection coatings, which require the use of wet layers with a thickness of 3 mm or more. On the one hand, such drying depends greatly on environmental conditions, especially on the relative humidity and on the temperature of the air. On the other, if a skin is formed quickly, the further drying of the film slows down due to the resulting vapor barrier. At the present time, accordingly, either long drying times are accepted or the systems are changed over to solvent-containing systems or systems having a high solids content. Such systems are based on clearly more expensive raw materials and, due to solvent vapors, represent a possible danger to man and the environment.
The German Publication DE 199 56 128 A1 describes fast drying, aqueous coating compositions, which contain an aqueous binder and at least one water-soluble, quaternary poly(allylamine) and, with that, exhibit accelerated drying properties without discoloring noticeably during the drying.
The European Patent EP 0 804 514 B1 already discloses a fast curing aqueous coating composition, which is used particularly as a paint for street markings and, very soon after it is applied on the surface of a traffic route, forms a hard, grease-resistant surface, which permits normal traffic to be resumed with a minimum interruption. This coating composition comprises 90 to 99.6% by weight of an anionically stabilized, aqueous emulsion of a copolymer of ethylenically unsaturated monomers, which contains a polyimine with a molecular weight of 250 to 20,000 and a volatile base. With such a coating composition, it becomes possible to produce fast drying coatings with a wet film thickness of only up to 0.5 mm.
However, such coating compositions are not suitable for producing thicker applied layers, particularly when the surfaces, which are to be treated, extend vertically, as is customary, in addition to the horizontal application, in the case of a fire-protection coating for a fagade application. Moreover, the rapid curing of previously known coating compositions is achieved by a rapid formation of a surface film, which, because of the decreased diffusion of water vapor through the surface film leads to unsatisfactorily long curing times in the case of thicker layers, which is serious especially when a vigorous rain, such as a driving rain occurs after the application of the coating composition and can cause components of the coating composition to wash or bleed out or the whole coating to wash off. Accordingly, the thin surface layer may be destroyed by the impact of the rain and the uncured material underneath may be washed off.
A further disadvantage of such physically curing coating compositions based on aqueous acrylate dispersions is seen to lie therein that, before they are cured, their resistance to driving rain is inadequate. Driving rain is defined as a heavy rain, the drops of which, depending on the drop size and the wind velocity, can result in damage to a coating composition, which has not yet cured, in that surface regions, which have not yet cured sufficiently, are washed out or beaten out of an application joint. This may lead to a bleeding of the fillers contained in the coating composition or, in the case of a breakdown, to a decreased strength of the coating film that finally cures. This is particularly serious for coating multistory facades, the lower coating regions of which, which have not yet been cured adequately, may be washed out in the event of a heavy rainfall. Both lead to a decreased tightness with respect to smoke or gas up to the tearing of the coating by movement of the treated surface with failure of the protective action resulting therefrom in the case of a fire.
The occurrence of driving rain must therefore be taken into consideration for the construction of building structures. Resistance to driving rain accordingly is understood to be the property of a surface to withstand this heavy rain permanently.
However, it is also of great importance to achieve a sufficiently large early resistance to driving rain, that is, the property of a physically curing coating composition based on aqueous acrylate dispersions to withstand the effect of such a driving rain even shortly after the application of the composition and before the complete curing thereof. Such water-based coating compositions, after the application thereof on the surface to be treated, pass through a period of time, the curing phase, during which the coating composition is water soluble and, accordingly, not resistant to driving rain. This is necessarily the case with the water-based systems normally used, since said systems are intended to be water-soluble during the processing and can easily be cleaned with water. Within the scope of the present invention, the period of time between this system-inherent lack of resistance to driving rain, from the application on the surface to be treated up to the time at which resistance to driving rain is achieved, is referred to as early resistance to driving rain. The shorter the early resistance to driving rain of such a water-based coating composition, the less is the risk of damage by any driving rain, which may occur, to a freshly applied coating before the latter has cured.